The present invention relates generally to a projection screen apparatus and more particularly to a control mechanism for a motor-driven projection screen apparatus.
While the size of the projection screen is not of the essence, these screens which are intended for use with the present invention are generally quite large, ranging in width from 5 to 14 feet, or more. They are the type of screens which are frequently found in auditoriums, in school rooms or in conference rooms. They are remotely controlled electrically by a small electric motor having a rotatable shaft which is operatively coupled to a large roller. The roller is generally journaled in a casing and the casing houses both the roller and the electric motor. The screen is, of course, rolled upon the roller when the screen is moved to a closed position and unrolled from the roller when it is moved to an exposed viewing position.
Generally, a master control switch is provided and is selectively operable to move the screen between open and closed positions. The master control switch may be moved to a first condition wherein the motor turns the roller to permit the screen to be played out from the roller or to a second condition wherein the motor reverses and turns the roller to wind the screen on the roller. Usually, the operator must carefully regulate the master control switch so that the motor is turned off just as soon as the screen is fully retracted or fully extended in order to prevent damage to the screen.
One device in the prior art which attempts to include an automatic shutoff, so as to eliminate the need for operator regulation, employs a worm gear on which a traveler nut having a pair of switches is movably disposed. As the roller rotates the traveler nut moves along the gear which rotates along with the roller. Also included in the device are a pair of adjustable actuators, each of which cooperates with one of the switches. As the traveler nut moves toward the end of the gear, the plunger on one of the switches is depressed by one of the actuators to terminate the operation of the motor. A significant problem of the device is adjusting the actuators so that the motor driving the roller stops at the proper time. It is necessary to determine the number of worm gear rotations corresponding to the extended and retracted position of the roller and, thereafter, the actuators are set. Because of the wriggle of the traveler on the gear and the tolerances in the actuators and the switch plungers, it is extremely difficult to achieve accurate and permanent adjustment of the device. Hence, positive and faultless termination of the screen movement may not always be achieved. The screen may retract or play out beyond a predetermined desirable position.